The low-head dam is the most dangerous type of dam and has been named the "drowning machine." They may not be easily spotted because the top of a low-head dam can be several feet below the water's surface. Because of their small size and drop, low-head dams do not appear to be dangerous. However, water going over a low-head dam creates a strong recirculating current or backroller (sometimes referred to as the "boil") at the base of the dam. Even on small rivers, the force of the backroller can trap your boat against the face of the dam and pull you under the water—even while wearing your PFD. Be aware that on large rivers or during high water the backroller or boil may be located more than 100 feet downstream of the dam. Avoid low-head dams.